Integrin-mediated signaling in the regulation of osteoclast adhesion and activation

Integrins are heterodimeric membrane receptors that mediate cell-extracellular matrix (ECM), and cell-cell interactions. Integrins provide a physical link between the ECM and the cell cytoskeleton, and transduce signals which lead to elevation of cytosolic pH and calcium levels, changes in phospholipid metabolism and ultimately regulate gene expression. Osteoclast bone resorption is a complicated multistep process, that starts with matrix recognition, osteoclast attachment, polarization and formation of the sealing zone on the bone, followed by the directional secretion of acids and lysosomal enzymes to the resorbing surface. Osteoclasts exhibit high expression of the alpha v beta 3 integrin, which binds to a variety of RGD-containing proteins including vitronectin, osteopontin and bone sialoprotein. RGD-containing peptides, RGD-mimetics and blocking antibodies to alpha v beta 3 integrins were shown to inhibit bone resorption in vitro and in vivo, suggesting that this integrin plays an important role in regulating osteoclast activity. Furthermore, RGD-containing peptides and proteins modulate osteoclastic cytosolic calcium levels. Phosphatidyl inositol 3-kinase and c-Src were co-immunoprecipitated with alpha v beta 3 integrins in these cells. In addition, c-Cbl was found to be a substrate of c-Src in osteoclasts. More recently, ligand-engagement or clustering of alpha v beta 3 integrins in osteoclasts induced tyrosine phosphorylation of PYK2, a member of the focal adhesion kinase family, and of p130cas, a substrate of v-Src and v-Crk. Both PYK2 and p130cas were also found in the sealing zone of actively resorbing osteoclasts. How these signaling molecules interact with each other in mediating the alpha v beta 3 rate limiting effect on bone resorption is not well understood. They emerged however as key players in linking the adhesion of osteoclasts to the bone matrix, to cytoskeletal organization, and to the polarization and activation of these cells for bone resorption.